It's less over-broad than ours in one very nice way: for every
legitimate style prop, it specifies the right type for its values
(right down to the allowed set of values for things like
`flexDirection`), and will cause the type-checker to complain on an
ill-typed value.

Without this change, after the upcoming upgrades Flow gives error
messages here.  If I understand the error messages correctly (though
I'm not confident I do), the point is that these particular components
insist on their props being read-only -- and while ViewStyleProp and
friends make that guarantee, DangerouslyImpreciseStyleProp doesn't.

These will be resolved properly when we eventually go about and narrow
down uses of DangerouslyImpreciseStyleProp to the more specific types.
Until then, just leave these fixmes.

Holding back from v0.57.2 because of issues there.

Skipping v0.56 because `jest` fails with a Babel config issue (with
or without the Babel config change in this commit), and at this point
v0.56 is old enough it's not worth trying to sort that out.

Both upgrade tools (react-native-git-upgrade and `react-native
upgrade`) failed to run, so instead this was a manual but thorough
old-school upgrade.  Not a problem, since we did that already for all
but the last upgrade.

Most of the changes required for the upgrade have been merged in a
variety of commits over the past few weeks; see zulip#2789.  This commit
 * upgrades `react-native` and closely related dependencies;
 * updates the Flow config file to match RN upstream's template; and
 * adjusts the Babel config to work now that most things use Babel 7:
   move the preset back to our specific babelrc, and re-enable babelrcs.

Fixes zulip#2788.

Typically we'd do this in tandem with the dependency upgrade, but
there's a lot of noise here and there are several substantive changes
in the parent commit; and, happily, the type-checker already passes at
the parent without this update.

Remove the $FlowFixMe comments as Flow no longer rises:
`Props has no `narrow` property`

The props we use to set the colors of the Switch component,
`onTintColor` and `tintColor`, are now deprecated.

Update to their replacement, the new `trackColor` property, setting
the exact same values to preserve the actual style unchanged.